Fixation and stabilization devices are known for the fixation of bone fractures or for the stabilization of the spinal column, which consist of at least two bone screws that are anchored in the bone or the vertebra and/or are connected via a plate or a rod. Such rigid systems do not permit any movement of the bone parts or vertebrae because they are fixed relative to each other.
For example, U.S. Pat. No. 5,474,555 describes an apparatus for internal fixation of the spine, with a screw element and a receiving part for connection to a rod, in which the screw element to be anchored in the bone is connected to the receiving part such that swiveling motions of the screw element and the receiving part are possible. However, the described solution does not allow for the mutual stabilization of bone parts and controlled partial motion.
However, for certain indications, a dynamic stabilization is desirable, in which the bone parts and vertebrae to be stabilized are able to carry out a controlled limited movement relative to each other. One option for realizing the dynamic stabilization device is the use of an elastic element instead of a rigid rod to connect the bone anchoring elements.
U.S. 2003/0109880 A1 describes a dynamic stabilization device for vertebrae is known, which comprises a first and a second screw to be anchored in the vertebra, each of which has a receiving part for the insertion of a spring connecting the screws, and a spring of this type. The spring itself, as a whole, is provided in the form of a coil spring with closely neighboring turns, similar to a tension spring and is fixed in the receiving parts by means of clamping screws. This, however, poses the risk that the spring, owing to its flexibility, evades the pressure of the clamping screw, thus causing the fixation between the bone screw and the spring to loosen. A further disadvantage of the device is that the elasticity of the spring, with otherwise identical spring characteristics, depends on the length of the spring.
EP 0 669 109 B1 discloses a stabilization device for stabilizing neighboring vertebrae, which comprises two monoaxial pedicle screws and a strap that is fixed in the receiving parts of the pedicle screws by means of a clamping screw, and which contains a pressure-resistant support element that is mounted on the strap. However, a stabilization device of this type fails to provide for stabilization against torsion. As in the stabilization device described above, the elasticity of the connection of the two bone anchoring elements, with otherwise identical characteristics of the spring elements, depends on the length of the spring coils and/or the spacing of the bone anchoring elements.
A joint fixation device, for example, for a wrist or a knee joint, is disclosed in U.S. Pat. No. 6,162,223. The apparatus comprises a fixation rod that is connected to bone anchoring elements at its ends and consists of two parts, wherein the two parts of the fixation rod are connected to one another via a flexible coupling and wherein the fixation rods and the coupling are arranged outside of the body. The two pieces of the fixation rod are not firmly connected to the coupling part, but rather can move freely along a bore hole in the coupling part. The ends of the two parts of the fixation rod that are facing each other are designed hemispherically and abut against each other, thus simulating a type of joint whose freedom of movement is limited by the flexible coupling. Owing to the type of connection to the two-piece fixation rod, the diameter of the coupling part is always larger than the diameter of the fixation rod. Owing to its complex and voluminous structure, this known joint fixation apparatus is not suitable for internal use on the spinal column or other bones.
U.S. 2003/0220643 A1 discloses a strechable element to be used in an apparatus for preventing full extension between upper and lower vertebral bodies.
U.S. Pat. No. 4,959,064 discloses a dynamic bone fixation screw for rejoining fracture fragments of a bone to their approximate original mutual dispositions. The screw includes a threaded distal end member for threaded engagement with one of the bone fragments and a head member at a proximal end for engagement with another of the bone fragments. A spring member is integral with and connects the threaded distal member with the head member. Alternative designs for dynamic bone fixation screws with a flexible shaft to stabilize a joint or a bone fracture are disclosed by EP 1 273 269 A2. However, none of these screws are connected with each other via a connecting member.
Therefore, there remains a need for bone stabilization devices that permit limited movement of vertebrae or bones that are to be connected to each other.